In recent years, advancement of the manufacturing technique of optical fiber base materials has increasingly enabled optical fiber base materials with a large outer diameter to be manufactured. To draw such optical fiber base materials with a large outer diameter into optical fibers having an outer diameter of 125 μm, the established drawing technique requires a considerable change. For example, a heating furnace of a larger size is required, and the drawing technique needs to be improved.
Considering the above problem, optical fibers are manufactured by using the following exemplary methods. An optical fiber base material with a large outer diameter manufactured by using the typical vapor phase axial deposition (VAD) method is first elongated so that the diameter is reduced to be substantially equal to the diameter of a typically-used optical fiber base material for drawing. This elongated optical fiber base material is drawn into an optical fiber. Alternatively, an optical fiber base material with a large outer diameter manufactured by using the VAD method is elongated so that the diameter is reduced. Subsequently, glass particles are deposited on the elongated optical fiber base material by using the outside vapor deposition (OVD) method, and the resulting optical fiber base material is drawn.
The above methods include a step of dehydrating a porous base material manufactured by depositing glass particles in a heating furnace and vitrifying the porous base material into a transparent glass. The vitrification step is normally performed in such a manner that while rotating, the porous base material is slowly moved through a furnace core tube the temperature of which is maintained at 1,000° C. to 1,650° C. Specifically speaking, the porous base material is hung such that the lower end portion of the porous base material is positioned in the vicinity of the heating source of the heating furnace. After the heating source is heated to the temperate at which the vitrification takes place, the porous base material is gradually moved down while being rotated. In this way, the vitrification proceeds from the lower end portion of the porous base material to the upper end portion.
When the porous base material is vitrified into a transparent glass, the density greatly changes in a portion of the porous base material which first experiences the vitrification. This may considerably affect the shape of the optical fiber base material obtained as a result of the vitrification depending on the position of the center of the rotation, the temperature distribution, the shape and the like. The obtained optical fiber base material may have a distorted portion as shown in FIG. 1.